1. Field of the Invention
The present invention relates to a coating field for forming a film, which contains, as the main component, a polysiloxane obtained by condensation polymerization of an alkoxy group-containing silicon compound, such as a coating fluid for forming a film, which is capable of forming a water repellent film, and a film formed from such a coating fluid and a process for forming such a film.
2. Discussion of Background
Heretofore, it has been common to form a protective film on a substrate surface as a method for imparting an abrasion resistance to a plastic substrate made of an acrylic resin, a polyester or a polycarbonate.
As a coating fluid to form such a protective film, a heat curable coating fluid containing, as the main component, a polysiloxane obtained by hydrolytic condensation polymerization of a silicon compound such as a tetraalkoxysilane, has been developed. However, with such a heat curable coating fluid, the heat treating temperature at the time of forming a film is restricted by e.g. the heat resistant temperature of the substrate, whereby there has been a case where the film is not sufficiently cured, and no adequate abrasion resistance can be obtained, and it has been desired to lower the heat treating temperature.
As a method for accomplishing such an object, for example, a method of lowering the temperature for forming a film has been proposed which utilizes a catalytic reaction by adding a curing catalyst such as a metal salt of an organic acid, a tin alkyl ester compound, a tin halide compound, a tin orthoester compound, a metal alcoholate, a titanium chelate compound or a nitrogen-containing basic organic compound (Patent Document 1), or a photocurable coating fluid requiring no heat treatment has been proposed which employs a photocurable resin composition comprising (A) an acrylate of bisphenol A diglycidyl ether polymer, (B) dipentaerythritol monohydroxy pentaacrylate, (C) a photopolymerization initiator, (D) inorganic particles and (E) a terminal reactive polydimethylsiloxane (Patent Document 2).
In recent years, plastic substrates, particularly plastic films, tend to have their thicknesses made thin for the purpose of high transparency, weight reduction, etc., and there has been a problem that they are likely to be damaged by heat. A photocurable coating fluid has been developed, but has been still inadequate. Accordingly, it has been desired more than ever to develop a heat curable coating fluid for forming a film which is sufficiently curable by such a low temperature treatment that a plastic film will receive no damage.
On the other hand, it has been known that if a film showing a refractive index lower than the refractive index of a substrate is formed on the surface of such a substrate, the reflectance of light reflecting from the surface of such a film will decrease. And, the film showing such a decreased light reflectance has been utilized as an antireflection film and has been applied to various substrate surfaces.
A process for forming on a substrate an antireflection film showing a low refractive index, is disclosed wherein an alcohol dispersion of fine MgF2 particles formed by reacting a magnesium salt or an alkoxymagnesium compound as a Mg source with a fluoride as a F source, or a liquid having a tetraalkoxysilane or the like added thereto for improvement of the film strength, is used as a coating fluid, and such a coating fluid is applied on a glass substrate such as a cathode ray tube and subjected to heat treatment at from 100 to 500° C. (Patent Document 3).
A low reflection glass having formed on a glass substrate a thin film showing a refractive index of from 1.21 to 1.40, having micropits or convexoconcaves with diameters of from 50 to 200 nm and having a thickness of from 60 to 160 nm, is disclosed, wherein a coating fluid is prepared by mixing at least two types of hydrolytic condensation polymers of tetraalkoxysilane, methyltrialkoxysilane, ethyltrialkoxysilane, etc. different in the average molecular weight with a solvent such as an alcohol, a film is formed from such a coating fluid by adjusting the mixing ratio in such mixing and by controlling the relative humidity, and then the film is heated (Patent Document 4).
A low reflectance glass is disclosed which comprises a glass, an underlayer film having a high refractive index formed on its surface, and an upper layer film having a low refractive index formed further on its surface (Patent Document 5). In this document, as a method for forming such an upper layer film, a method is disclosed wherein a fluorinated silicon compound having a polyfluorocarbon chain such as CF3(CF2)2C2H4Si(OCH3)3 and from 5 to 90 wt %, based thereon, of silane coupling agent such as Si(OCH3)4 are hydrolyzed in an alcohol solvent in the presence of a catalyst such as acetic acid at room temperature, followed by filtration to prepare a liquid of a co-condensate, and then, such a liquid is applied on the above-mentioned underlayer film and heated at from 120 to 250° C.
It is disclosed that a reaction mixture comprising a silicon compound represented by Si(OR)4, a silicon compound represented by CF3(CF2)nCH2CH2Si(OR1)3, an alcohol represented by R2CH2OH and oxalic acid in a specific ratio, is heated in the absence of water at from 40 to 180° C. to form a polysiloxane solution, a coating fluid containing such a solution is applied on a substrate surface, and the applied film is heat-cured at from 80 to 450° C. to form a film as adhered to the substrate surface, which has a refractive index of from 1.28 to 1.38 and a contact angle with water of from 90 to 115° (Patent Document 6).
In recent years, along with the progress in weight reduction or thickness reduction of display devices, an antireflection substrate, particularly an antireflection film, to be mounted thereon, is desired to have the film thickness made thin for the purpose of weight reduction, high transparency or the like, whereby there has been a problem that the film is susceptible to a damage by heat. Accordingly, it has been desired more than ever to develop a heat curable coating fluid for forming a film, whereby an antireflection substrate can be obtained by such a low temperature treatment that the film is not susceptible to a damage.
Patent Document 1: JP-A-07-082486
Patent Document 2: JP-A-11-124514
Patent Document 3: JP-A-05-105424
Patent Document 4: JP-A-06-157076
Patent Document 5: JP-A-61-010043
Patent Document 6: JP-A-09-208898